The present invention relates generally to pneumatic tires and more specifically to pneumatic tires designed to be used primarily off-the-road, and even more specifically as agricultural (farm) tires or tractor tires. Still more specifically the invention relates a tread having good soil cleaning characteristics especially in the removal of mud.
Pneumatic tires designed primarily for off-the road use often feature an "open" tread pattern with a lower net-to-gross ratio than tires intended for highway use. For example the net-to-gross ratio of off-the-road or agricultural tires can range from 25% to 40% as compared to about 60-75% for on the road tires.
Off-the-road or agricultural tires also commonly feature axially extending tread lugs which cut into the loose ground surface and an inner tread surface, the area between the protruding tread lugs, which usually contacts the ground surface. In normal operation, the lugs cut into the loose ground surface and provide a means for axial traction and lateral stability while the inner tread surface provides flotation and supports the weight of the vehicle. In a tire designed for a more conventional highway application, the area of the inner tread surface between the protruding tread lugs is not meant to contact the road surface.
In that these tires are capable of being used on a variety of surfaces, such as mud, sand, compacted soil, gravel, etc. the foreign matter or soil must be "cleaned", removed, from between the lugs, otherwise the axial traction and lateral stability will be reduced. In other words, if the grooves, the area between the lugs, becomes compacted or filled during a rotation of the tire by soil and it is not removed during the next rotation, the traction for subsequent tire rotations will be reduced. This may then result in the tractor or vehicle becoming immobile.
In order to enhance the cleaning or removal of soil or other debris from the tread, attempts have been made to place cleaning elements between the lugs, such as U.S. Pat. No. 3,030,998 to Jensen. The cleaning elements extend along the axial or lateral extent of the lugs and appear to be as tall as the lugs themselves. It is believed that this attempt to solve the problem of cleaning focuses on the wrong area of the tread. Rather, it is believed that the area between the lugs should remain as free and clear of obstruction as possible while placing the cleaning element at the discharge area of the tire, ie the lateral edges of the tread. In other words, if the area adjacent to the lateral edges of the tread remain open, the soil will be able to be discharged from between the lugs as it is channeled therefrom. The placing of objects, or ribs, such as taught in U.S. Pat. No. 3,030,998 is believed to hinder the natural flow or discharge of the soil.